This book constitutes the refereed proceedings of the 20th International Conference on Formal Engineering Methods, ICFEM 2018, held in Gold Coast, QLD, Australia, in November 2018. The 22 revised full papers presented together with 14 short papers were carefully reviewed and selected from 66 submissions. The conference focuses on all areas related to formal engineering methods, such as verification; network systems; type theory; theorem proving; logic and semantics; refinement and transition systems; and emerging applications of formal methods.